The intestinal mucosal surface is continuously exposed to microbial communities and intestinal homeostasis is dependent on host barrier function and the establishment and persistence of well- regulated immune responses. Innate immune cells play critical roles in the maintenance of mucosal homeostasis by establishing immune tolerance. A recent study identified an inflammatory bowel disease (IBD) causal sub-network of genes (IL10, ARC, NOD2, HCK, WAS, NCKAP1L), which were highly enriched in bone-marrow derived macrophages and predicted to have a role in anti-inflammatory macrophage function. Wiskott-Aldrich syndrome protein (WAS) was one of the genes identified within this sub-network, along with HCK, a protein that interacts with Wiskott-Aldrich syndrome protein (WASP). Moreover, up to 10% of Wiskott-Aldrich syndrome patients develop IBD. We have shown that WASP-deficiency in mice, when limited to innate immune cells, is sufficient to render normal wild-type CD4+ T cells colitogenic with associated defects in tolerogenic dendritic cell (DC) subsets. While WASP may have regulatory functions in DCs, most exciting is our preliminary data presented within this proposal showing that WASP-deficiency is associated with marked abnormalities in anti-inflammatory macrophage differentiation and function. We found that WASP-deficiency is associated with defects in generation of tolerogenic macrophage in both mice and humans leading to aberrant pro-inflammatory cytokine secretion. Another integral part of the innate immune system involved in maintaining mucosal homeostasis is group 3 innate lymphoid cells (ILC3). These cells secrete IL-17 and IL-22, which are important for barrier integrity and have also been shown to regulate intestinal tolerogenic DC and M? populations, and CD4+ T cell responses to commensal bacteria. We found that in the absence of WASP, ILC3 function is aberrant. Therefore, We hypothesize that WASP-mediated regulation of anti- inflammatory M? and ILC3 function is critical for the maintenance of intestinal homeostasis. To test this we will first delineate the 1) intrinsic role of WASP in regulating anti-inflammatory M? function and define the mechanism of immune tolerance; second, we will uncover the 2) intrinsic role of WASP in ILC3 and their associated regulation of intestinal homeostasis; and finally we 3) investigate the role of WASP in anti-inflammatory M? and ILC3 development/function in humans.